A metamorphic process that causes snow grains to become angular with weaker bonds

Faceting is caused by strong vapor pressure gradients in the snowpack which form as a result of strong temperature gradients. Water vapor moves from regions of high vapor pressure to regions with low vapor pressure. This movement of water vapor changes the shape of snow grains to squares, cups, or other angular shapes which don’t bond as well to surrounding grains. This process most commonly occurs near the snow surface, around crusts, or near the ground. Sometimes the entire snowpack can facet during prolonged cold spells or in a shallow snowpack. 

Facets under magnification. Credit: The International Classification for Seasonal Snow on the Ground / AINEVA UniMilano